This invention relates to electric radiant heater units of the kind used in "smooth top" cookers. More particularly, it relates to such heaters which employ two or more heater elements in the same unit.
A "smooth top" cooker is one in which a smooth top normally of glass ceramic, overlies one or more generally circular electric heater elements supported on a layer of thermal and electrical insulating material such that the element is spaced from the top. In use, a utensil placed on the top over an element is heated by the transmission of heat from an element to and through the top by air convection, conduction and infra red radiation. Such elements are referred to as "radiant heaters". The insulating material substantially prevents heat being transmitted away from the element except towards the top and as the preferred materials for the top are essentially thermally non-conductive, only areas of the top which ae directly exposed to the element will be heated. In order to prevent heat being transmitted to parts of the top not covered by a utensil placed thereon, a peripheral wall of insulating material is also normally provided around the coil.
It is usual, and in some jurisdictions mandatory in radiant heaters to include a thermal cut-out device to protect both the element and the top from overheating. While it is possible to design a heater with a low watts density to obviate the need for a cut-out device, this leads to a slow cooking performance. Thus, a thermal cut-out device is desirable from both the point of view of safety and that of performance. Further, excessive temperatures can result in damage to or discoloration of the top in a smooth top cooker. For example, a glass ceramic top can discolour if the temperature at the exposed surface exceeds 600.degree. C. (700.degree. C. at the surface nearest the heater element or elements).